Alumina microspheres are produced according to various sol-gel techniques which generally involve creating an alumina hydrosol, e.g. by the action of dilute acetic acid on aluminum metal, and dispersing droplets of the gel into a water-immiscible solvent where the surface tension of the droplets draws the droplets into a sphere. The spheres of alumina hydrosol are congealed, e.g., by introducing the hydrogel spheres into ammonium hydroxide, and dried. The dried microspheres are calcined to split water molecules from the hydrated aluminum oxide and leave solid microspheres of alumina. When calcined at temperatures between about 400.degree. C. to about 1000.degree. C., the resulting microspheres are composed of activated alumina having very small crystallite size, high porosity and high surface area. As used herein, the term "activated alumina" encompasses alumina of a variety of crystallite structures having high porosity resulting in a surface area of at least about 125 m.sup.2 /g. (The term "surface", as used herein in reference to the activated alumina, includes the surfaces of the interior pores as well as the exterior surfaces of the alumina). Activated alumina may take several crystallite forms including delta, gamma, kappa and, most commonly, theta alumina. For purposes of this invention, activated alumina having a crush strength of at least about 25 Kpsi is of primary interest. Porous activated alumina is highly hygroscopic and is commonly used as a highly effective desiccant.
Porous activated alumina has a low density and is quite resilient resulting in a high crush strength, i.e., as high as 45 Kpsi at a density of 1.4 g/cm.sup.3. Their low density and high resiliency suggest the use of activated alumina microspheres for such applications as proppants for gas well fracturing, catalyst support, granular bed filtration, heat transfer media, etc. However, activated alumina is a relatively unstable form of alumina which may be degraded by contact with moisture, even water vapor in the air, and by elevated temperatures. Activated alumina microspheres exposed to the atmosphere at 20.degree. C. and at 50% relative humidity for 24 hours will absorb sufficient moisture to reduce its crush strength from 30 Kpsi to 12 Kpsi. At temperatures over 1100.degree. C., activated alumina is converted to .varies.-alumina which is less resilient than activated alumina and has very little crush strength. For example, .theta.-alumina microspheres with a crush strength of 27 Kpsi heated at 1200.degree. C. for 30 minutes are converted to a .varies.-alumina with a crush strength of less than 2 Kpsi. Accordingly, raw activated alumina is unsuitable for many purposes which involve the presence of water or water vapor and has limited use as a refractory material.
It would be desirable to have a method of treating activated alumina, particularly activated alumina microspheres, to waterproof the alumina and to increase its thermal stability while retaining the desirable properties of low-density, and high crush strength. In particular, it would be desirable to prevent the conversion of activated alumina to low porosity, low crush strength .varies.-alumina.